The invention relates to a manually operated lung-venting apparatus comprising a self-expanding bladder of the type having an inlet in one end wall and an outlet in the other end wall. The bladder by periodical compression and release enables breathing gas to be supplied into the lungs of a patient.
Such lung-venting systems fundamentally can belong to one of two different types, that is, open or closed systems. In the open system the pressure source is normally a so-called self-expanding venting bladder, i.e. a bladder which after compression automatically resumes its normal shape due to its inherent resiliency. The self-expanding bladder is provided with an inlet having a one-way suction valve and an outlet in communication with a three-way breathing valve. The outlet of the breathing valve is in communication with the lungs of the patient via a breathing mask or the like. When the bladder is compressed the suction valve is closed and the gas contained in the bladder is discharged through the breathing valve and via the breathing mask into the respiratory ducts of the patient. When the bladder is released after an insufflation process, it is re-filled with fresh gas through the suction valve while the patient is exhaling whereafter a new inhalation may be performed.
Closed lung-venting systems fundamentally comprise a closed circuit through which the breathing gas is flowing in a given direction with the aid of suitable one-way valves. In this case too, a compressible venting bladder is used in order to perform the insufflation, but the bladder, in this case, need not be completely self-expanding but may be adapted to be filled after a compression with fresh gas to some extent due to the pressure conditions prevailing in the system which is supplied with fresh breathing gas from a source of gas which is continously connected to the system.
In these and similar lung-venting systems it is necessary to protect the lungs of the patient against excessive venting pressures while at the same time care must be taken to supply the lungs under all conditions with adequate amounts of gas. In known systems the lungs ordinarily are protected by the provision of an excess pressure valve adapted to open at a predetermined pressure valve thereby to permit gas to be discharged from the system thus reducing the pressure therein.
An essential disadvantage of such systems resides in that the gas is discharged from the system when the excess pressure valve is opened which may mean that the remaining gas quantity is not sufficient to meet the patient's requirements. This condition is of particularly great importance in case the patient exhibits increased air duct resistance in front of the alveoli because the pressure drop across the air duct resistance may mean that the pressure within the system exceeds the opening pressure of the excess pressure valve before the alveoli have been sufficiently filled with gas; thus there is a great risk of an insufficient breathing-air supply to the patient.
Another disadvantage with these known systems comprising an excess pressure valve resides in that the treating gas is discharged into the environment which, on the one hand, means wastage of treating gas and, on the other hand, requires the provision of means for eliminating the outflowing treating gas which may be noxious.
The problems in connection with volume losses due to discharge of the treating gas into the environment through the excess pressure valve are of particularly great importance if a self-expanding venting bladder is used for lung-venting purposes because such bladders are characterized in that only a limited maximum pumping volume is at hand, which means that volume losses cannot be compensated for by unlimited increase of the pumping volume. As such self-expanding venting bladders, in addition, are of simple construction, it is not possible to measure in a simple way the proportion of the total pumping volume actually received within the lungs of the patient in comparison with the volume of the gas escaping into the environment. Thus the evaluation of the volume received by the lungs of the patient must be made in accordance with a subjective assessment which will yield fairly accurate results only after long experience.